Pass variables to replace tokens in query

[rafatower]

This fixes #218 along with a minor change in node-mapnik

What this does:

• it optionally adds a new member mapnik::attributes vars to the mvt processor.
• it is passed by processor::update_tile to the tile_layer, so that it can set the variables of its associated query from there (empty if not passed to the processor constructor).

(thanks @Algunenano for the advice on the constructor)

This is the patch that would be required in node-mapnik to indeed pass the variables to the datasources. I haven't created a PR yet because it makes little sense until this one approved.

[codecov-io]

Codecov Report

Merging #248 into master will increase coverage by 0.69%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   64.08%   64.77%   +0.69%     
==========================================
  Files          30       30              
  Lines        2230     2308      +78     
==========================================
+ Hits         1429     1495      +66     
- Misses        801      813      +12

Impacted Files	Coverage Δ
src/vector_tile_processor.hpp	0% <0%> (ø)	:arrow_up:
src/vector_tile_processor.ipp	74.85% <100%> (+0.14%)	:arrow_up:
src/vector_tile_layer.hpp	86.36% <100%> (-13.64%)	:arrow_down:

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[springmeyer]

Thanks for this @rafatower! Nice work - looks great. Will merge now.